The disproportionation or metathesis of olefins is a reaction in which one or more olefinic compounds are transformed into other olefins of different molecular weights. The disproportionation of an olefin to produce one olefin of a higher molecular weight and one olefin of a lower molecular weight can also be referred to as a self-disproportionation. For example, propene can be disproportionated to ethylene and cis- and trans-2-butene. Another type of disproportionation involves the codisproportionation of two different olefins to form still other olefins. For example, the reaction of one molecule of 2-butene with one molecule of 3-hexene can produce two molecules of 2-pentene.
Several catalyst systems have been proposed for the disproportionation of olefins. Several disadvantages have been observed for these catalyst systems. In some cases, isomerization of the double bond of the starting material or product occurs and disproportionation involving the isomeric olefin yields a mixture of products which is difficult to separate. With certain catalysts, polymerization of the olefins occurs at long reaction times or high reaction temperatures. Some catalysts cause alkylation of aromatic solvents with the olefin, thereby consuming some of the reactant or product and producing a more complex product mixture. Some catalysts are only effective for terminal olefins and other catalysts may be effective only with internal olefins. Many of the metathesis catalyst systems use expensive organoaluminum compounds or other organic complexes which present operational difficulties during production, storage, and use.
Some olefins have very close boiling points and, thus, are very difficult to separate by conventional means, such as distillation. For example, 4-methyl-1-pentene and 4-methyl-2-pentene produced by potassium on potassium carbonate catalyzed dimerization of propylene have boiling temperatures of 53.9.degree. C. and 56.3.degree. C., respectively. The separation of thermodynamically more stable isomer 4-methyl-2-pentene, which is lack of industrial use, from 4-methyl-1-pentene which is a monomer for poly(4-methyl-1-pentene), a high temperature polyolefin, is, therefore, very difficult by simple distillation.
There is, therefore, a need to develop a catalyst that is very effective in olefin disproportionation. There is also a need to develop a process to simplify and facilitate the separation of industrially important olefins by simple distillation from other olefins that have very close boiling points.